Optical lithography has advanced in recent years in its ability to produce very small features. The most important recent advance in lithography was the introduction of a photomask structure called a phase shift mask. Phase shift masks enable compensation for the diffraction effects which otherwise limit the size of the smallest feature which could be imaged using optical lithography.
Optical lithography refers generally to the technology which enables etching patterns on a substrate through use of photographic development of images which have been attached onto the surface of the substrate using a mask. Generally, the process involves directing light (such as ultraviolet light) through a photomask to expose a light-sensitive film previously deposited on the substrate. If the lightsensitive film is a so-called positive resist and the resist is located beneath a clear area in the photomask, the resist undergoes a physical and chemical change that renders it soluble in a development solution. This process results in the transfer of an image from the photomask to the resist film. Finally, application of an acid to the surface will transfer the resist image into the surface of the substrate.
There are several known types of photomasks for use in lithography which employ phase shifting. The underlying concept of a phase shift mask is to introduce canceling interference of impinging light at portions of an image where diffraction effects have deteriorated the resolution of the image. This is accomplished by providing a mask with appropriately placed and appropriately selected thicknesses of optically transmissive materials so that the ultraviolet light waves which pass through the mask and then image on the target exhibit constructive phase addition at areas in which high intensity for imaging is desirable, and destructive phase subtraction where low intensity is desired. Constructive and destructive phase is explained by considering light as a wave motion so that the effect of a number of wave trains arriving at a point depends on the phase as well as on the amplitude of each of the arriving waves. If light from the same point source starts in phase and travels different paths through different materials to come together at a point, if the waves arrive in phase, they will reinforce each other, i.e., constructive interference. If light from more than one source arrives at the same point, if the various source emissions are coherent, i.e., same polarization, frequency and phase, then their waves can also combine constructively or destructively at the image. Accordingly, for phase shift masks to be effective, it is required that the light source emits light which is at least partially coherent.
A most significant application of phase shift masks (PSMs) in optical lithography is in the formation of electrical circuits on semiconductor materials in the manufacturing of integrated circuits such as semiconductor memory devices, microprocessors and other circuits. Other applications include the manufacture of compact discs and other laser readable memory devices. In the manufacture of integrated circuits, the apparatus most frequently employed to cooperate with the phase shift mask to image the phase shift mask onto the semiconductor substrate is called an optical stepper. The optical stepper positions and holds a wafer and photomasks provide the partially coherent ultraviolet light to image the various photomasks onto the wafer. Generally, the optical system, including the mask, is stationary and an image of a device is formed on a substrate positioned at the focal plane. Normally, the optical stepper is capable of moving the substrate horizontally, i.e., in steps, and permits the same optical exposure imaging process for the adjoining devices. The most common commercial steppers are called "I-line Steppers" where "I" designates that the wavelength of the ultraviolet source lamp being used, i.e., .lambda.=365 nm. A more advanced stepper employs a deep-uv source where .lambda.=250 nm.
Exemplary PSMs use phase shift materials which transmit about 8% of the energy incident on the mask. This level of background illumination does not usually lead to undesirable levels of resist loss in the exposed wafer. However, if certain geometries, such as dense contact arrays, or L- or T-shaped patterns are found on the mask, then the intensity at the wafer may increase locally for example to 30% of clear-field exposure. Such a high level of illumination may lead to resist loss, causing the printing of sidelobes.
Referring to FIG. 1, a prior art illumination system 10 includes a mask 12 used to selectively illuminate portions of a wafer 14 covered with a photoresist material 16. The mask 12 includes a light transmissive substrate 20, with a partially-transmissive phase-shifting material 22 on portions of the substrate 20. The phase-shifting material is a material which absorbs most of the light passing therethrough, and shifts the phase of the light which it does allow to pass therethrough. Light passing through the phase-shifting material in phase-shifting regions 24 is preferably shifted in phase by approximately 180 degrees, thereby making it opposite in phase in comparison with light that passes through portions of the mask 12 which do not have a coating of phase-shifting material 22, such as the light transmissive or open region 26 shown in FIG. 1. Light 28 passes through the mask 12 and exposes the resist 16 on the wafer 14.
The transmissivity of the mask 12 is plotted in FIG. 2, wherein the transmissivity of the open region 26 is represented as a positive value 30 and the transmissivity of the regions 24 with phase-shifting material 22 is represented as a negative value 32. The negative value for the transmissivity in regions of the mask 12 covered by the phase-shifting material 22 indicates the interference between light passing through open region 26 and light passing through phase-shifting regions 24.
The electric field intensity from the light 28 reaching the resist 16 is illustrated in FIG. 3, the electric field intensity from the open region 26 being shown by a curve 36, and the electric field intensity from the phase-shifting regions 24 being shown by a curve 38. The open region curve 36 is generally positive, with sinusoidal end portions 40 away from the open region 26, the end portions having asymptotically-reducing amplitude. The phase-shifting region curve 38 has a negative electric field intensity, the negative value being constant far from the open region 26, and reducing to zero in the vicinity of the transition between the phase-shifting regions 24 and the open region 26.
The curves 36 and 38 are summed and squared to give the light exposure in the resist 16, as illustrated in FIG. 4. The light exposure profile has a main peak 42 corresponding to the center of the open region 26. The light intensity drops off from the main peak to main troughs 44 on either side of the main peak 42. Moving further away from the main peak 42 are secondary peaks 46 and tertiary peaks 48. Far away from the main peak 42 is a constant exposure 50 which corresponds to the electric field intensity through phase-shifting regions far away from an open region. A printing threshold intensity level, shown in FIG. 4 as dashed line 54, is the minimum intensity level required for sufficient exposure of the resist 16 to eventually result in printing on the wafer 14. As illustrated, the intensity level results in printing a feature having a width 56 which is less than the width of the open region 26.
It will be appreciated that the threshold 52 must be higher than the level of the secondary peaks 46 in order to avoid printing on the wafer due to the secondary peaks 46. However, a secondary peak may combine with a secondary or tertiary peak from another feature to locally exceed the printing threshold. These undesired areas where the intensity exceeds the threshold are referred to as "sidelobes," "additive sidelobes," or "proximity effects."
It has been proposed to use simple geometric rules to place extra features on the mask to suppress sidelobes. However, these rules are arbitrary and will be incomplete and may be inadequate to prevent sidelobes in all cases.
Further, the use of geometric rules may result in more extra features than are necessary to suppress side lobes.